The present invention relates to nuclear reactor fuel assemblies, and more particularly, to apparatus and method for fabricating nuclear fuel assembly grids.
The engineering and fabrication of nuclear fuel assemblies has become a highly developed art, which takes into consideration the physics associated with the fission process, the structural strength and thermal hydraulic characteristics, and ease of fabrication. Superficially, nuclear fuel assemblies from most commercial vendors appear similar in that they consist of an eggcrate formed by the interengagement of slotted zircaloy strips so as to define a plurality of individual cells having integrally formed fuel rod support structure projecting therein. Subtle design differences can, however, result in significant differences in in-reactor performance and cost of fabrication.
One such detail is shown in the fuel assembly of U.S. Pat. No. 4,879,090, the disclosure which is hereby incorporated by reference. FIG. 1 of the present application reproduces one of the Figures in the '090 patent, to show vanes projecting inwardly towards the center of each cell. These projections are generally referred to as mixing vanes and are intended to promote the swirling and cross flow of coolant within and between flow chambers, so as to maintain a substantially uniform coolant temperature at any given elevation in the grid during operation of the reactor. For thermal hydraulic performance considerations, these vanes should be relatively large. From the point of view of minimizing labor cost for fabricating the grids, the vanes should be bent relative to the upper edge of each strip, before the strips have been assembled into the eggcrate configuration.
For reasons to be described more fully below, however, the geometric relationship between large vanes and the fuel rod support structure which projects from each strip, and the necessity of forming the eggcrate by lowering one type of strip onto another type of strip so that the respective slots will be interengaged, has posed difficulties arising from the interaction of the vanes with the fuel rod support structure during fabrication. The relatively large, rigid fuel rod support structure interferes, or hangs up on the bent vanes of the neighboring strips as the strips are moved vertically relative to each other during fabrication.
Increasing the projection of mixing vanes in the direction normal to their bend axis increases their blockage area and improves their DNB performance. It also moves the vane tips into the path of the rod supports on the perpendicular strips as the latter slide past during assembly. Increasing the width (the dimension perpendicular to the fuel rod axis) of the rod supports tends to reduce the plastic deformation of the material from which type from which they are formed. This minimizes the potential for cracking or "orange peeling" during the stamping operation but it also increases the width of the path that they sweep out during assembly. The residual stress induced by the formation of the rod supports (especially the arch) in the slotted portion of the strip causes the tabs between slots to bow out of the plane of the strip. When these tabs contain mixing vanes that bow can increase the intrusion of the vane tips into the path of the rod supports and further aggravate interference between them. The vane/rod support interference can greatly complicate the assembly of the individual strips into a spacer grid array but after the strips are fully meshed, it ceases to be a problem.
One conventional approach is either to minimize labor cost but suffer a performance penalty by utilizing small vanes that, even when bent prior to fabrication, will not interfere with the fuel rod support structure during fabrication. The other compromise solution is to maximize in-reactor performance by providing relatively larger vanes, but at the increased labor costs associated with bending the vanes after the eggcrate has been assembled.